There is known a vehicular drive system including a differential mechanism operable to distribute an output of an engine to a first electric motor and an output shaft, and a second electric motor disposed between the output shaft of the differential mechanism and drive wheels. Patent Document 1 discloses an example of such a vehicular drive system in the form of a hybrid vehicle drive system. In this hybrid vehicle drive system, the differential mechanism is constituted by a planetary gear set, for example, and a major portion of an output of the engine is mechanically transmitted to the drive wheels through the differential function of the differential mechanism, while the other portion of the output of the engine is electrically transmitted from the first electric motor to the second electric motor through an electric path, so that the differential mechanism functions as a transmission the seed ratio of which is electrically variable, for instance, as an electrically controlled continuously variable transmission. The hybrid vehicle drive system is controlled by a control device such that the vehicle is driven with the engine kept in an optimum operating state for improving the fuel economy.    Patent Document 1: JP-2000-2327 A    Patent Document 2: JP-2001-234775 A
Generally, a continuously variable transmission is known as a device for improving the fuel economy of the vehicle, while a gear type power transmitting device such as a step-variable automatic transmission is known as a device having a high degree of power transmitting efficiency. However, there has not been available a power transmitting mechanism which has advantages of those two devices. For instance, the hybrid vehicular drive system disclosed in the above-identified Patent Document 1 has an electric path for transmitting an electric energy from the first electric motor to the second electric motor, namely, a power transmitting path through which a portion of the vehicle drive force is transmitted as the electric energy, so that the first electric motor must be large-sized with an increase of the nominal engine output, and the second electric motor which is operated by the electric energy generated by the first electric motor must also be large-sized. Accordingly, the drive system tends to be unfavorably large-sized. Alternatively, a portion of the engine output is once converted into an electric energy, which is subsequently used to drive the drive wheels, so that there is a risk of deterioration of the fuel economy in some running condition of the vehicle, for example, during a high-speed running of the vehicle. There is a similar problem where the power transmitting mechanism as described above is used as a transmission the speed ratio of which is electrically controlled, for example, as a continuously variable transmission called “an electric CVT”.
It is known that the output torque of the engine (hereinafter referred to simply as “engine torque”) varies or fluctuates due to periodic ignition (explosion) of its cylinders, or switching between a lean-burn state in which the air-fuel ratio of the engine is higher than the stoichiometric value, and a rich-burn state in which the air-fuel ratio is lower than the stoichiometric value.
As described in Patent Document 2, for example, an operation of the engine in the lean-burn state for improving the fuel economy of the engine causes an increase of concentration of oxygen contained in the exhaust gas, resulting in promotion of absorption of NOx by an NOx absorbent provided in the exhaust passage. Since the NOx absorbent is limited in its capacity to absorb NOx, a so-called “rich spike” is implemented to temporarily switch the engine from the lean-burn state to the rich-burn state, that is, to the fuel-rich state, for discharging the absorbed NOx from the NOx absorbent so that the NOx absorbent is restored to its state for effective absorption of NOx. The rich spike results in an increase of the fuel supply to the engine, and a consequent temporary increase (variation) of the engine torque.
If the engine torque variation is transmitted to the drive wheels, there arises a variation of the vehicle drive torque, giving rise to a risk of deterioration of the drivability of the vehicle. It is desired to reduce the amount of the engine torque variation to be transmitted to the drive wheels. For instance, a vehicular drive system including a torque converter with a lock-up clutch disposed between an engine and an automatic transmission is arranged to place the lockup clutch in a slipping or released state, for thereby causing the torque converter to absorb the engine torque variation, to reduce the amount of the engine torque variation to be transmitted to the drive wheels.
In the drive system as disclosed in Patent Document 1 which is not provided with a fluid-operated power transmitting device such as the torque converter, however, the engine torque variation may be transmitted to the drive wheels through a power transmitting path connecting the engine and the drive wheels, leading to a risk of deterioration of the drivability of the vehicle due to the engine torque variation. The vehicular drive system which solves the problem of the hybrid vehicle drive system also suffers from a risk of deterioration of the engine torque variation.
The present invention was made in view of the background art described above. It is an object of this invention to provide a control apparatus for a vehicular drive system including a differential mechanism capable of performing a differential function to distribute an output of an engine to a first electric motor and an output shaft, and an electric motor disposed in a power transmitting path between the differential mechanism and a drive wheel, which control apparatus permits size reduction of the drive system or improvements of fuel economy and drivability of a vehicle.